The gasification of carbonaceous material produces primarily carbon monoxide and hydrogen, mixture known as syngas. Carbon dioxide, water and various hydrocarbons are abundant side products in the gasification product. Depending on the source and composition of the carbonaceous raw material and gasification conditions, the levels of side products and derivatives typically present as impurities vary influencing the refining strategies.
During gasification, the sulfur and its derivatives originated from biomass are mainly converted to hydrogen sulfide (H2S) and carbonyl sulfide (COS). In comparison to coal gasification, gasifying biomass raw material produces very low levels of sulfidic, relatively low levels of nitric and low levels of ashes impurities. The level of carbon dioxide is typically higher than in coal gasification. These impurity levels are still harmful for further chemical processing and the gas must be purified. The decrease of hydrogen sulfide concentration is compulsory for the functioning of the catalysts later in the refining of the syngas. On the other hand, the carbon dioxide's role in the further reactions is basically as an inert. The reason for removing CO2 relates to optimizing the streams and decreasing volumes of recycle flows and equipment. The strategies known from coal gasification are not readily applicable.
Together carbon dioxide, hydrogen sulfide and carbonyl sulfide are referred to as acid gas since they dissolve in water forming acids. One of the most common means for gas purification is absorption, which has been used for acid gas removal from natural and synthesis gases. When purifying biomass originated synthesis gas, absorption with a liquid solvent has shown to be more efficient than solid absorption. For physical absorption, organic solvents at cold temperatures and high pressure are common. Roughly, the higher the pressure, the colder the temperature and higher the purity of the absorbent, the better is the washing effect. For chemical absorption, solutions of arsenic salts, various amines and carbonates are known. Generally, the absorbent is regenerated by rising the temperature and/or releasing the pressure.
Prior art discloses effective absorbents for removing acid gas using e.g. methanol. Methanol requires low temperatures to be efficient and to avoid absorbent loss. A very well-known commercial process using methanol is desulfurization process marketed under trade name Rectisol®. The Rectisol desulfurization process does not require hydrolysis of COS to H2S and can reduce sulfur compound contents to relatively low levels in syngas. Methanol has a high affinity for hydrocarbons as well as for acid gas. It also exhibits capabilities to remove not only sulfur compounds and CO2 but also many relevant trace components (carbonyles, HCN), which makes Rectisol wash a useful process. The syngas is then reheated to about 350° C. and passed through a fixed bed of a sorbent for sulfur compounds, such as a ZnO guard bed, to further reduce the sulfur compound contents in the syngas. Large temperature differences between process phases consume a lot of energy and makes processing expensive.
In prior art, document EP 2223889 discloses a device providing further development of the multistage methanol wash as a part of Integrated Gasification Combined Cycle, IGCC. With the device disclosed, as a multistage process, this version of Rectisol process removes CO2 as well from the gas. As a process related to power production, the purity requirements are, however, different from those applied in chemical or fuel production wherein higher purity is demanded.
Another document of prior art, US 2010163803, discloses a process for the production of gas products from a raw synthesis gas that is obtained by gasification of carbon and/or heavy oil. Origin of the gas gives it a characteristic component profile. The process description discloses how both the shifted and the unshifted gas streams are purified of sulfur components and CO2 in sour gas washing, more specifically a cryogenic methanol washing. An apparatus suitable for the process is disclosed as well. Both sulfur components and CO2 are removed together, the washes providing no separation of these components.
In addition to physical absorption described above, chemical absorption is known in the art. Gas containing large volumes of hydrogen sulfide can be freed from said hydrogen sulfide by first conducting the gas stream into aqueous solutions containing copper ions in water for absorbing the hydrogen sulfide and then oxidizing the copper sulfide thus formed with air or oxygen gas to produce elemental sulfur. Prior art document DE 2304497 discloses an aqueous absorption medium which contains rather high concentrations of copper ions (28.9 g Cu in 1400 ml water), and absorption of the hydrogen sulfide carried out by bubbling the gas into the aqueous medium.
Another document representing prior art, EP0986432 B1, discloses a method for selective hydrogen sulfide removal from gases comprising both H2S and CO2. When these components were present in the gas in CO2 to H2S ratio of 2:1, the method removed 99% of the H2S selectively. However, when said ratio was 200:1, the H2S removal was 95%.
There still is a need for an alternative method for removal of sulfur components and carbon dioxide from syngas obtainable by gasification of carbonaceous material, especially when gasifying biomass. Further, there is a need to remove sulfur components and carbon dioxide from the syngas in an energy efficient way. There also is a need for an effective combined sulfur component and carbon dioxide removal. Yet, there is constant need for simplification, increase of the effectiveness and identification of possibilities for synergism of the overall BTL process.